Method and system for recovery of methane from hydrocarbon streams

ABSTRACT

The invention relates to a method for recovery of methane from hydrocarbon streams comprising the following steps: a. Introducing a feed fluid stream (F), which comprises methane fluid, at least one hydrocarbon free fluid, wherein in particular said at least one hydrocarbon free fluid is nitrogen, and at least one hydrocarbon fluid, into a demethanizer system ( 1 ); b. Separating said feed fluid stream (F) in the demethanizer system ( 1 ) into a carbon rich fraction (C), comprising hydrocarbons with a carbon content of C2 and higher, and a separation stream (S), comprising methane fluid and at least one hydrocarbon free fluid; c. Introducing said separation stream (S) into a hydrocarbon-free fluid separation system ( 2 ), in particular in a cryogenic hydrocarbon-free fluid separation system ( 2 ′), more particularly into a cryogenic nitrogen rejection system ( 2 ″); wherein said separation stream (S) is compressed by a compressor system ( 6 ) before said separation stream (S) is introduced in said hydrocarbon-free fluid separation system ( 2 ), wherein said separation stream is compressed to a pressure of 12 bar to 80 bar; d. Separating said separation stream (S) in said free fluid separation system ( 2 ) into a methane stream (M) and a hydrocarbon-free fluid stream (HF) and a respective system for recovery of methane from hydrocarbon streams.

The invention relates to a method for recovery of methane from hydrocarbon streams and a system for recovery of methane from hydrocarbon streams.

Methane is a very important natural gas which is used in a huge variety of different applications. One important use of methane is as a fuel, since burning methane produces less carbon dioxide for each unit of heat released in comparison with other hydrocarbon fuels. Usually methane is supplied in the form of a liquefied natural gas (LNG) for storage and transportation purposes. Another very important use of methane is the application of methane as a reactant in a technical synthesis. Methane is an important starting material, for example, for the technical synthesis of hydrogen, methanol, ethylene, hydrogen cyanide, methyl halogenides, or organic compounds.

In general, such technical grade syntheses yield a synthetic gas mixture (reaction mixture) comprising different reaction products, unreacted starting materials and, optionally, other compounds which were introduced during the reaction process, but did not participate in the reaction itself. Different methods have been developed in order to separate the target product, or the target products, from the reaction mixture. Commonly a demethaniser is applied in order to separate methane and other hydrocarbon-free compounds (e.g. hydrogen or nitrogen) from the remaining hydrocarbon compounds in the reaction mixture, which comprise a carbon content of C₂ or higher.

The use of a demethaniser system on a synthetic gas mixture aims at the separation of methane and other hydrocarbon-free gases from a reaction mixture in order to facilitate a further subsequent separation step of the now methane-free hydrocarbon fraction comprising hydrocarbons with a carbon content of C₂ or higher. The thus separated methane content is generally discharged or has to be further processed in order to be used for any further process or synthesis.

In this regard, US 2013/225884A1 discloses processes for producing and separating ethane and ethylene, wherein an oxidative coupling of methane (OCM) product gas comprising ethane and ethylene is introduced to a separation unit comprising two separators. Within the separation unit, the OCM product gas is separated to provide a C₂-rich effluent, a methane-rich effluent, and a nitrogen-rich effluent.

Given that the availability of methane (and other natural resources) is limited and the worldwide demand is increasing it is problematic that the unreacted reactant methane cannot be reused, in particular for further synthesis purposes, without an extensive treatment prior of said reuse.

This problem is solved by a method comprising the features of the independent claim 1 and an integrated system comprising the features of the independent claim 13 which allow a recovery of methane from hydrocarbon streams, wherein the recovery of methane particularly allows the facile recycling and reuse of the methane content for further synthesis purposes.

The method of the invention for the recovery of methane from hydrocarbon streams comprises the following steps:

-   -   a. introducing a feed fluid stream, which comprises a methane         fluid, at least one hydrocarbon-free fluid, wherein in         particular said at least one hydrocarbon-free fluid is nitrogen,         and at least one hydrocarbon fluid, into a demethaniser system;     -   b. separating said feed fluid stream in said demethaniser system         into         -   a carbon-rich fraction, which comprise hydrocarbons with a             carbon content of C₂ and higher, and         -   a separation stream, which comprises methane fluid and at             least one hydrocarbon-free fluid;     -   c. introducing said separation stream into a hydrocarbon-free         fluid separation system, in particular in a cryogenic         hydrocarbon-free fluid separation system, more particularly into         a cryogenic nitrogen rejection system; wherein preferably said         separation stream is compressed by a compressor system before         said separation stream is introduced in said hydrocarbon-free         fluid separation system, wherein preferably said separation         stream is compressed to a pressure of 25 bar to 80 bar;     -   d. Separating said separation stream in said free fluid         separation system into a methane stream and a hydrocarbon-free         fluid stream.

The method of the invention allows the provision of an essentially pure methane stream and a good separation of said methane stream from the feed fluid stream, which may be used in a reaction process for further products.

According to the invention, the term “feed fluid stream” is to be understood as a liquid and/or a gas stream comprising liquid or gaseous methane, liquid or gaseous hydrocarbon compounds, and/or a hydrocarbon-free fluid in liquid and/or gaseous form. According to the invention the term “hydrocarbon compounds” is to be understood as compounds with a carbon content of C₂ or higher which comprise at least one hydrogen-carbon bond. Such hydrocarbon compounds are particularly alkane or alkene compounds like ethane, ethane (ethylene), propane or propene (propylene) and the like.

According to the invention the term “hydrocarbon-free fluid” is to be understood as a compound in a liquid or a gaseous form which comprises no hydrogen-carbon bond, such as hydrogen, nobel gases, CO, CO₂, or nitrogen. A hydrocarbon-free fluid is particularly argon, CO, hydrogen or nitrogen, more particularly nitrogen.

In some embodiments said feed fluid stream derives from a synthesis system which uses methane as a reactant. Such synthesis systems may be a system designated for the oxidative coupling of methane (OCM) or a methane pyrolysis. In some embodiments the synthesis system is a system designated for the oxidative coupling of methane (OCM).

The oxidative coupling of methane is a known chemical reaction (OCM reaction) applied to the conversion of methane into further chemicals, in particular into ethan, ethylene, C3-hydrocarbons or C4-hydrocarbons, more particularly ethylene. The reaction is generally carried out in the presence of a catalyst and comprises several reaction and separation steps for producing ethylene from a methane feed. The methane feed is generally mixed with compressed air and comprises after the reaction with the catalyst nitrogen, methane, CO, CO₂, hydrocarbons with a carbon content of C₂ or higher (e.g. ethan, ethylene, C3-hydrocarbons or C4-hydrocarbons), and water.

The principle product of OCM is ethylene, the world's largest commodity chemical, and the fundamental building block of the chemical industry. However, methane activation is difficult owing to its thermodynamic properties. This limits the efficient utilisation of methane, an important petrochemical resource. The application of a catalyst in the reaction system and the adjustment of the reaction conditions have improved the conversion of methane in an OCM reaction. However, the products of OCM reactions—depending on the reaction conditions—may react to undesired by-products. In order to improve the selectivity of the products, such as ethylene, a low conversion of methane is used. Thus, a significant amount of unreacted methane is left in the reaction mixture.

In some embodiments, said methane stream is recycled and reused as a reaction product in a technical synthesis.

In some embodiments, the feed fluid stream is derived from a synthesis system, which uses methane as a reactant and said feed fluid stream is separated in said demethaniser system into said carbon-rich fraction and said separation stream, wherein said separation stream is introduced into said hydrocarbon-free fluid separation system, in particular in said cryogenic hydrocarbon-free fluid separation system, more particularly into said cryogenic nitrogen rejection system, and wherein said separation stream is separated in said hydrocarbon-free fluid separation system into a methane stream and a hydrocarbon-free stream.

In some embodiments, the feed fluid stream is derived from a synthesis system designated for an OCM reaction and said feed fluid stream is separated in said demethaniser system into said carbon-rich fraction and said separation stream, wherein said separation stream is introduced into said nitrogen rejection system, in which said separation stream is separated into a methane stream and a nitrogen stream.

In some embodiments, said separation stream is compressed by said compressor system before said separation stream is introduced in said hydrocarbon-free fluid separation system, wherein in particular said separation stream is compressed to a pressure of 25 bar to 75 bar, preferably to a pressure of 25 bar to 60 bar, more preferably to a pressure of 25 to 40 bar, more preferably to a pressure of 30 to 40 bar, particularly to a pressure of 30 bar. The boundaries of the above pressure ranges may also be combined in an arbitrary fashion. Furthermore, in some embodiments, the lower pressure boundary of these pressure ranges may also be one of: 12 bar, 13 bar, 14 bar, 15 bar, 16 bar, 17 bar, 18 bar, 19 bar, 20 bar, 21 bar, 22 bar, 23 bar, 24 bar, 25 bar, 26 bar, 27 bar, 28 bar, 29 bar.

The compression of the separation stream after leaving this demethaniser system and before the introduction into the hydrocarbon-free fluid separation system allows for a better separation and isolation of methane and hydrocarbon-free gas, in particular nitrogen, from the separation stream as in comparison to a direct introduction of the separation stream from the demethanizer system into the hydrocarbon-free fluid separation system.

As discussed previously, a higher pressure in the hydrocarbon-free fluid separation system allows for a better separation of methane and the hydrocarbon-free gas. However, a lower pressure is preferred in the demethaniser system, since the increase of pressure in the demethaniser would lead to higher hydrocarbon product losses. For example, if the feed fluid stream is derived from an OCM separation system, too high a pressure would lead to ethylene product losses. The use of a compressor system, in particular of a 3-step compressor system (e.g. a compressor system described in the document WO02/088612A1) allows for a compensation of the preferably lower pressure in the demethaniser system and allows the separation of the separation stream in the hydrocarbon-free fluid separation system at a higher pressure.

In some embodiments the separation stream derived from the compressor is cooled down, in particular the separation stream is cooled down in a plate fin heat exchanger, and expanded to a lower pressure, before said separation stream is introduced into the hydrocarbon-free fluid separation system.

In some embodiments, said carbon-rich fraction from the demethaniser system is transferred to a C2-splitter in which hydrocarbons with different carbon contents of said carbon-rich fraction are separated from each other. The use of a C2-splitter allows separation and isolation of target products from the carbon-rich fraction. C2-splitters are known in the art, and the dimensions and separation conditions depend on the target compound, which depends in itself on the previously-applied synthesis system, which provides the feed fluid stream. For example, if an OCM reaction system is used, which provides the feed fluid stream, the C2-splitter is designed and operated in such a way that the target compound ethylene can be separated in high purity. If the separated methane stream (as described previously) is recycled and reintroduced into the OCM reaction system (synthesis system), the target compound ethylene could be achieved in a more cost-effective way, since the (nowadays restricted) natural resource methane is used more efficiently.

In some embodiments, said carbon rich fraction of the demethanizer system is reboiled in a reboiler, in particular said carbon rich fraction of the demethanizer system is reboiled before said carbon rich fraction is transferred to said C2-splitter.

In some embodiments, at least parts of the feed fluid stream are liquidized in a cooling system before the introduction into a demethaniser unit of said demethaniser system. The demethaniser unit is designed to separate methane and the hydrocarbon-free fluid, in particular nitrogen, from the hydrocarbons with a carbon content of C₂ or higher from the reaction mixture derived from the reaction system. A demethaniser unit may be for example a distillation column.

In some embodiments, said feed fluid stream is separated in said cooling system into a liquid feed fluid stream and a gaseous feed fluid stream, wherein said liquid feed fluid stream is transferred to said demethaniser unit and said gaseous feed fluid stream is transferred to a (first) expander booster system, in which said gaseous feed fluid stream is expanded to a lower pressure before introducing into said demethaniser unit. The expansion of the gaseous feed fluid stream in the expander booster system allows recovery of work power, which might be used in a subsequent compression step in the demethaniser system.

In some embodiments, the liquid feed fluid stream from the cooling system and the gaseous feed fluid stream from the (first) expander booster system are combined in said demethaniser unit and separated in the demethaniser unit into a carbon-rich fraction and separation stream. After the separation the separation stream is introduced into a second expander booster system, in which it is expanded, in particular expanded to approximately 4 bar, before said separation stream is introduced in said hydrocarbon-free fluid separation system. The expansion of the separation stream provides the chilling duty used in the demethaniser system.

In some embodiments, said gases feed fluid stream is introduced into a first expander booster system in which said gases feed fluid stream is expanded to a lower pressure before introducing into said demethaniser unit. Furthermore, said separation stream from said demethaniser unit is introduced in the second expander booster system in which it is expanded to provide said chilling duty. Additionally, the work power of both expanders is recovered to recompress the separation stream, in particular to recompress the separation stream to approximately 6 bar, before said separation stream is introduced into said hydrocarbon-free fluid separation system.

In some embodiments, the demethaniser system is operated at a pressure of 6 to 40 bar.

In some embodiments, the demethaniser unit of said demethaniser system is operated at a pressure of 9 to 25 bar, in particular at a pressure of approximately 13 bar. In some embodiments the demethaniser unit is operated at a temperature range of −20 to −170° C. In some embodiments, the demethaniser unit comprises a degrading temperature range along its longitudinal axis, wherein in particular the demethaniser unit comprises a temperature of −30° C. at the bottom of the demethaniser unit, and a temperature of approximately −150° C. at the top of the demethaniser unit.

In some embodiments, said separation stream is introduced in at least one high-pressure column arranged in said hydrocarbon-free fluid separation system, in which said separation stream is separated in a methane-rich bottom liquid and an essentially pure gaseous hydrocarbon-free overhead, wherein said methane-rich bottom liquid is transferred into at least one low-pressure column arranged in said hydrocarbon-free fluid separation system, in which said methane-rich bottom liquid is separated in hydrocarbon-free gas and a methane-rich liquid fraction. The methane-rich liquid fraction is at least partially vaporized—providing a liquid fraction and a methane gas fraction—wherein the cold derived from said liquid methane gasification is used for the separation process. Thus, allowing for the separation of a hydrocarbon-free gas fraction and a methane gas fraction, wherein both fractions are discharged from the low-pressure column.

In some embodiments, the low-pressure hydrocarbon-free gas fraction and the liquid fraction—derived from the partial vaporization of said methane-rich bottom liquid—are used to cool the inlet streams of both columns.

In some embodiments, said methane-rich bottom liquid is transferred to the mid part of said low-pressure column.

In some embodiments, said methane-rich bottom liquid and said gaseous hydrocarbon-free overhead are sub cooled in a cooler, particularly a reflux cooler, to approximately −160° C. before they are transferred to said low-pressure column.

In some embodiments, said hydrocarbon-free overhead from the high-pressure column is at least partially condensed and said bottom liquid from the low-pressure column is at least partially vaporised on a heat exchanger, in particular on a heat exchanger which is arranged between said high-pressure column and said low-pressure column.

In some embodiments, said at least one high-pressure column and said at least one low-pressure column are integrated in one unit, wherein said at least one high-pressure column and said at least one low-pressure column are interconnected with a heat exchanger situated between both columns.

The use of a high-pressure column and a low-pressure column, particularly connected with a heat exchanger situated between both columns, allows for the separation of the separation stream into an essentially pure hydrocarbon-free gas and an essentially pure methane gas. In a preferred embodiment the feed fluid stream is provided from an OCM reaction system, thus comprising a very high content of nitrogen and a substantial amount of methane. The method of the invention allows firstly the separation and isolation of the methane and nitrogen mixture (separation stream) from the reaction mixture of the OCM reaction (the feed fluid stream) in said demethaniser system, and secondly the separation from each other in said cryogenic nitrogen rejection system in a very high purity. Thus, the gaseous methane can be recycled and reused in the OCM reaction system.

In some embodiments said high-pressure column is operated at a pressure of 6 to 40 bar, in particular at a pressure of approximately 20 bar, and a temperature of −160 to −90° C., in particular at a temperature of approximately −140° C., and wherein said low-pressure column is operated at a pressure of 1 to 5 bar, in particular at a pressure of approximately 2 bar, and a temperature of −220 to −180° C., in particular at a temperature of approximately −190° C.

The use of the aforementioned separation conditions, in particular use of such a high pressure in the hydrocarbon-free fluid separation system, allows for a good separation of methane from a hydrocarbon-free gas.

According to another aspect of the invention the invention comprises a system for recovery of methane from hydrocarbon streams comprising:

-   -   a. a demethaniser system, which is designated to separate a feed         fluid stream, which comprises methane fluid, at least one         hydrocarbon-free fluid, wherein in particular said at least one         hydrocarbon-free fluid is nitrogen, and at least one hydrocarbon         fluid, into         -   a carbon-rich fraction, which comprises hydrocarbons with             carbon content of C₂ and higher, and         -   a separation stream, which comprises methane fluid and at             least one hydrocarbon-free fluid,     -   b. a hydrocarbon-free fluid separation system, in particular a         cryogenic hydrocarbon-free fluid separation system, more         particularly a cryogenic nitrogen rejection system, which is         designated to separate said separation stream into a methane         stream and a hydrocarbon-free stream; and     -   c. preferably a compressor system that is configured to compress         said separation stream to a pressure of 12 bar to 80 bar         upstream said hydrocarbon-free fluid separation system.

In some embodiments, the compressor system is configured to compress said separation stream to a pressure of 15 bar to 75 bar, preferably to a pressure of 20 bar to 60 bar, more preferably to a pressure of 25 bar to 40 bar, more preferably to a pressure of 30 bar to 40 bar, particularly to a pressure of 30 bar, before said separation stream is introduced in said hydrocarbon-free fluid separation system.

The boundaries of these pressure ranges may also be combined in an arbitrary fashion. Furthermore, in some embodiments, the lower pressure boundary of these pressure ranges may also be one of: 12 bar, 13 bar, 14 bar, 15 bar, 16 bar, 17 bar, 18 bar, 19 bar, 20 bar, 21 bar, 22 bar, 23 bar, 24 bar, 25 bar, 26 bar, 27 bar, 28 bar, 29 bar.

In some embodiments the system of the invention comprises a synthesis system, which uses methane as a reactant and provides said feed fluid stream, wherein in particular said synthesis system is a system for oxidated coupling of methane (OCM), wherein in particular the system comprises means to transfer the recovered and isolated methane from the hydrocarbon-free fluid separation system to the synthesis system.

Concerning further embodiments, reference is made to the detailed description of the method of the invention and the figures.

Further details and features of the invention are described in the following figures of two embodiments of the invention.

FIG. 1 shows a first embodiment of the invention comprising a demethaniser system 1 and a hydrocarbon-free fluid separation system 2; and

FIG. 2 shows a second embodiment of the invention comprising a demethaniser system 1, a cryogenic nitrogen rejection system 2″, and an OCM synthesis system 3.

FIG. 1 shows a system for recovery of methane from hydrocarbon streams comprising a demethaniser system 1 and a hydrocarbon-free fluid separation system 2.

A feed fluid stream F, comprising methane fluid, at least one hydrocarbon-free fluid, and at least one hydrocarbon fluid is introduced into a demethaniser unit 10 of the demethaniser system 1. The demethaniser unit 10 is operated at a pressure of 13 bar. Different pressures may be applied as necessary.

The demethaniser unit 10 comprises a temperature gradient with a temperature of −30° C. at the bottom of the demethaniser unit 10 and a temperature of approximately −150° C. at the top of the demethaniser unit 10. Thus the demethaniser unit 10 allows for a separation of the feed fluid stream F into a carbon-rich fraction C at the bottom of the demethaniser unit 10, and a separation stream S, which comprises methane fluid and at least one hydrocarbon-free fluid, in particular nitrogen, at the top of the demethaniser unit 10.

Optionally the feed fluid stream F may be cooled down with at least one cooling system (not depicted in the figure), wherein each separated liquid of each cooling step (liquid feed stream) is introduced into the demethaniser 10. A remaining gaseous feed stream may be transferred from the cooling systems into an expander boost system (not depicted in the figure), in which it is expanded to a lower pressure and subsequently introduced into the demethaniser unit 10.

The carbon-rich fraction C from the bottom of the demethaniser 10 is reboiled in a reboiler 4 in order to provide a carbon-rich fraction C, which is free of methane and hydrocarbon-free fluids like nitrogen. The carbon-rich fraction C is then transferred to a C2 splitter 7 for further separation in order to isolate the target product from the carbon-rich fraction C. For example, the target product is ethylene if the feed fluid stream F is derived from a synthesis system 3 (see FIG. 2), which applies the oxidative methane-coupling reaction (OCM).

The separation stream S is then transferred from the top of the demethaniser unit 10 to the hydrocarbon-free fluid separation unit 2. Optionally the separation stream S may be transferred—prior to the introduction to the hydrocarbon-free fluid separation system 2—into a second expander (not depicted), where it is expanded to approximately 4 bar, providing the chilling duty used in the demethaniser system. The work power of the first and the second expander can be recovered in order to recompress the separation stream S to approximately 6 bar, before it is introduced into the hydrocarbon-free fluid separation system 2.

The hydrocarbon-free fluid separation system 2 comprises a high-pressure column 21 and a low-pressure column 22, which are interconnected with a heat exchanger 5 situated between the high-pressure column 21 and the low-pressure column 22. Before the separation stream S is introduced into the bottom of the high-pressure column 21 it may be cooled down by, for example, a plate fin heat exchanger.

Alternatively, the high-pressure column 21 and the low-pressure column 22 can be constructed as separate columns.

In the high-pressure column 21 the separation stream S is separated into a methane-rich bottom liquid at the bottom of the high pressure column 21 and a gaseous stream, comprising essentially pure hydrocarbon-free overhead product, in particular an essentially pure nitrogen overhead product. The pressure at the bottom of the high-pressure column 21 is approximately 20 bar, and the temperature is approximately −140° C. The bottom liquid from the bottom of the high pressure column 21 is transferred to the mid-section of the upper low-pressure column 22.

Optionally the bottom liquid may be sub-cooled in a reflex cooler to approximately −160° C. before it is transported to the mid-section of the low-pressure column 22. The low-pressure column 22 operates at a pressure of 2 bar, which allows for a further separation of hydrocarbon-free gas, in particular nitrogen, and methane, due to their physical properties.

Columns 21 and 22 are connected by an integrated heat exchanger 5. In this heat exchanger 5 the overhead vapour from the high-pressure column 22 will be condensed while simultaneously the bottom liquids from the lower-pressure column 22 will be partially vaporised. The low-pressure hydrocarbon-free gas, in particular nitrogen, and the methane can be used to cool the inlet streams of both columns. The use of the high-pressure column 21, the low-pressure column 22 and the integrated exchanger 5 allows for the separation and isolation of a hydrocarbon-free gas HF, in particular nitrogen, and methane M in a high purity. Alternatively, the high-pressure column 21, the low-pressure column 22 and the integrated exchanger 5 may be separate units.

The hydrocarbon-free product HF, in particular nitrogen, can be sent to the atmosphere, while the isolated methane M can be recycled and introduced into a reaction process, which uses methane as a reactant. Alternatively, hydrocarbon-free product HF and the isolated may be further processed before being sent to the atmosphere or being recycled and introduced into a reaction process.

FIG. 2 shows a system for recovery of methane from hydrocarbon streams comprising a demethaniser system 1, a cryogenic nitrogen rejection system 2″ and a synthesis system 3, which uses methane in an OCM reaction.

Concerning the description and the features of functions or applications with the same numbering or letter, reference is made to the description of FIG. 1. The system for recovery of methane from hydrocarbon streams is essentially the same as in FIG. 1.

The two main differences are that the feed fluid stream F derives from a synthesis system 3 which uses methane as a reactant in an OCM reaction. Thus, the separation stream S comprises essentially methane and nitrogen. Another difference is that, before the separation stream S is transferred from the demethaniser system 1 and introduced into the cryogenic nitrogen rejection system 2″, the separation stream S is compressed with a compression system 6 to approximately 25 bar to 80 bar, preferably to a pressure of 25 bar to 75 bar, preferably to a pressure of 25 bar to 60 bar, more preferably to a pressure of 25 to 40 bar, in particular to 30 bar. The other pressure ranges stated above may also be used.

As discussed previously, a cryogenic nitrogen rejection system 2″ provides a very good separation and isolation of nitrogen and methane, if it is operated with a high pressure.

Conversely the demethaniser system 1 is preferably operated at a lower pressure in order to minimise product losses concerning the main product ethylene (derived from the OCM reaction). Thus, the use of a compressor system 6, in order to provide a separation stream S with a higher pressure compared to the situation in the demethanizer system 1, compensates for these deficiencies.

The use of the feed fluid stream F derived from an OCM reaction, the separation of the feed fluid stream F in a demethaniser unit 10 into a carbon-rich fraction C and a separation stream S, the compression of said separation stream S, the subsequent separation of the compressed separation stream S in a cryogenic nitrogen rejection system 2″ into essentially pure nitrogen and essentially pure methane, and the recycling and re-use of the thus separated methane in the aforementioned OCM reaction allows for an efficient and economically effective use of the important reactant methane.

LIST OF REFERENCES

demethaniser system 1 demethaniser unit 10 hydrocarbon-free fluid separation system 2 cryogenic hydrocarbon-free fluid separation system 2′ cryogenic nitrogen rejection system 2″ high-pressure column 21 low-pressure column 22 synthesis system 3 Reboiler 4 heat exchanger 5 compression system 6 C2 splitter 7 hydrocarbon-free fluid stream HF methane stream M feed fluid stream F separation stream S carbon rich fraction C 

1. A method for recovery of methane from hydrocarbon streams comprising the following steps: a. Introducing a feed fluid stream (F), which comprises methane fluid, at least one hydrocarbon free fluid, wherein in particular said at least one hydrocarbon free fluid is nitrogen, and at least one hydrocarbon fluid, into a demethanizer system (1); b. Separating said feed fluid stream (F) in the demethanizer system (1) into a carbon rich fraction (C), comprising hydrocarbons with a carbon content of C₂ and higher, and a separation stream (S,) comprising methane fluid and at least one hydrocarbon free fluid; c. Introducing said separation stream (S) into a hydrocarbon-free fluid separation system (2), in particular in a cryogenic hydrocarbon-free fluid separation system (2′), more particularly into a cryogenic nitrogen rejection system (2″); wherein said separation stream (S) is compressed by a compressor system (6) before said separation stream (S) is introduced in said hydrocarbon-free fluid separation system (2), wherein said separation stream is compressed to a pressure of 25 bar to 80 bar; d. Separating said separation stream (S) in said free fluid separation system (2) into a methane stream (M) and a hydrocarbon-free fluid stream (HF).
 2. The method according to claim 1, wherein said feed fluid stream (F) derives from a synthesis system (3), which uses methane as a reactant, in particular said synthesis system (3) is a system for oxidative coupling of methane.
 3. The method according to any one of the claims 1 to 2, wherein said methane stream (M) is recycled and reused as a reactant, wherein in particular said methane stream (M) is transferred to said synthesis system (3).
 4. The method according to any of the previous claims, wherein said separation stream (S) is compressed by said compressor system (6) to a pressure of 25 bar to 75 bar, preferably to a pressure of 25 bar to 60 bar, more preferably to a pressure of 25 bar to 40 bar, particularly to a pressure of 30 bar, before said separation stream (S) is introduced in said hydrocarbon-free fluid separation system (2).
 5. The method according to claim 1, wherein said carbon rich fraction (C) from the demethanizer system (1) is transferred to a C2-splitter (7), for separation and isolation of hydrocarbon compounds with different carbon contents of said carbon rich fraction (C) from each other.
 6. The method according to any one of the claims 1 to 5, wherein at least parts of the feed fluid stream (F) are liquidized in a cooling system before the introduction into a demethanizer unit (10) of the demethanizer system (1).
 7. The method according to any one of the claims 1 to 6, wherein said feed fluid stream (F) is separated in said cooling system into a liquid feed fluid stream and a gaseous feed fluid stream, wherein said liquid feed fluid stream is transferred to said demethanizer unit (10) and said gaseous feed fluid stream is transferred to an expander-booster system, in which said gaseous feed fluid stream is expanded to a lower pressure before introducing said gaseous feed fluid stream into said demethanizer unit (10).
 8. The method according to any one of the claims 1 to 7, wherein said demethanizer system (1) system is operated at a pressure of 6 to 40 bar.
 9. The method according to any one of the claims 1 to 7, wherein said demethanizer unit (10) of said demethanizer system (1) is operated at a pressure of 9 to 25 bar, in particular at a pressure of approximately 13 bar.
 10. The method according to claim 1, wherein said separation stream (S) is introduced in at least one high pressure column (21), which is arranged in said hydrocarbon free fluid separation system (2), and in which said separation stream (S) is separated in a methane rich bottom liquid and an essentially pure hydrocarbon-free overhead, wherein said methane rich bottom liquid is transferred into at least one low pressure column (22), which is arranged in said hydrocarbon free fluid separation system (2), in which said methane rich bottom liquid is separated into a methane rich liquid and hydrocarbon-free gas.
 11. The method according to claim 10, wherein said hydrocarbon free overhead from the high pressure column (21) is at least partially condensed on a heat exchanger (5) and said a methane rich liquid from the low pressure column (22) is at least partially vaporized on said heat exchanger (5), providing a liquid fraction and a methane gas fraction, wherein said heat exchanger (5) is situated between said high pressure column (21) and said low pressure column (22).
 12. The method according to any one of the claims 10 to 11, wherein said high pressure column (21) is operated at a pressure of 6 to 40 bar, in particular at a pressure of approximately 20 bar, and at a temperature of −160 to −90° C., in particular at a temperature of approximately −140° C., and wherein said low pressure column (22) is operated at a pressure of 1 to 5 bar, in particular at a pressure of approximately 2 bar, and at a temperature of −220 to −180° C., in particular at a temperature of approximately −190° C.
 13. A system for recovery of methane from hydrocarbon streams comprising a. a demethanizer system (1), which is designated to separate a feed fluid stream (F), which comprises methane fluid, at least one hydrocarbon-free fluid, wherein in particular said at least one hydrocarbon-free fluid is nitrogen, and at least one hydrocarbon fluid, into i. a carbon rich fraction (C), which comprises hydrocarbons with a carbon content of C₂ and higher, and ii. a separation stream (S), which comprises methane fluid and at least one hydrocarbon free fluid, and b. a hydrocarbon-free fluid separation system (2), in particular a cryogenic hydrocarbon free fluid separation system (2′), more particularly a cryogenic nitrogen rejection system (2″), which is designated to separate said separation stream (S) into a methane stream (M) and a hydrocarbon free stream (HF), and c. a compressor system (6) that is configured to compress said separation stream (S) to a pressure of 25 bar to 80 bar before said separation stream (S) is introduced in said hydrocarbon-free fluid separation system (2).
 14. The system according to claim 13, wherein the compressor system (6) is configured to compress said separation stream (S) to a pressure of 25 bar to 75 bar, preferably to a pressure of 25 bar to 60 bar, more preferably to a pressure of 25 bar to 40 bar, particularly to a pressure of 30 bar, before said separation stream (S) is introduced in said hydrocarbon-free fluid separation system (2).
 15. The system according to claim 13 or 14, wherein said system comprises a synthesis system (3), which uses methane as a reaction educt and provides said feed fluid stream (F), wherein in particular said synthesis system (3) is a system for oxidative coupling of methane. 